Since IEEE (The Institute of Electrical and Electronics Engineers, Inc.) specified 802.11 standards in 1999, wireless LAN (Local Area Network) system is widely adapted in various applications.
In the wireless LAN system, due to retransmission of packets caused by packet losses, a problem that a throughput of the entire communication system may degrade occurs.
For example, in the case that the distance between a transmission source node and a destination node is far or a radio wave condition is unstable, packets losses may occur due to failure of a reception of the packets at the destination node. At the destination node, in the case that a reception condition of the datum from the transmission source node is poor, even if the transmission source node retransmits a packet, a possibility that the destination node repeats the failure in receiving the packet is high. As the result, in this case, the throughput of a radio communication system may degrade remarkably.
As a technology of settling the stated problem in a radio communication, a technology that selects an interception node in neighborhood of the destination node with a good communication condition between the destination node as a proxy transmission node of the destination node, and that the proxy transmission node retransmits the packet to the destination node instead of from the transmission source node is proposed.
Hereinafter, in the specification, “transmission source node” denotes a node which is an originating transmission source of transmission datum, “destination node” denotes a node which is a destination of the datum, and “interception node” denotes a node which can intercept the datum which are transmitted from the transmission source node to the destination node, respectively. In general, the interception node is located in the neighborhood of both the transmission source node and the destination node.
Patent document 1 describes a technology of retransmission packets using proxy transmission nodes. The patent document 1 discloses a configuration of the radio communication system wherein an interception node intercepts a communication between a transmission source node and a destination node, and in the case that the interception node cannot receive ACK (ACKnowledgement) message from the destination node, then the interception node retransmits a datum to the destination node.
Patent document 2 discloses a path control system that a transmission source node compares a quality of communication between the transmission source node and a destination node and a quality of communication between the destination node and a proxy transmission node, and selects a highest-quality path.
In addition, non-patent document 1 discloses a configuration of a wireless packet network that determines AIFS (Arbitration Inter Frame Space) and CWmin (Contention Window minimum) which are parameters in CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method, according to quality of communication between a relay node and a destination node, and selects an interception node having a higher quality of communication as a proxy transmission node with a high probability.
The operation of the interception node involved in the proxy transmission will be described referring to FIGS. 14 to 16. FIG. 14 is a configuration diagram of the wireless LAN system in which the interception nodes are located. FIG. 15 is a figure showing a normal retransmission operation of the wireless LAN system that the interception node does not retransmit. FIG. 16 is a figure showing a retransmission operation where the interception node retransmits as the proxy transmission node.
In the wireless LAN system shown in FIG. 14, a transmission source node 101 transmits a packet to a destination node 102. Cases are described in the following where the proxy transmission is and is not executed by the interception node in the wireless LAN system shown in FIG. 14.
FIG. 15 shows a normal retransmission operation of the wireless LAN system wherein the interception node does not execute the proxy transmission.
After transmitting a first transmission packet to the destination node 102, the transmission source node 101 waits for a reply using an ACK packet which indicates a reception confirmation from the destination node 102. However, in the case that the packet can not reach the destination node 102 correctly because of any reasons on the way, the destination node 102 does not transmit the ACK packet to the transmission source node. In the case that the transmission source node 101 does not receive the reply in the ACK packet from the destination node 102 until the fixed waiting time (Tw1) has passed after the transmission of the packet, the transmission source node 101 retransmits the transmission packet to the destination node 102.
FIG. 16 is a sequence chart showing the retransmission operation in the case that the interception node executes the proxy transmission as the proxy transmission node. A case where a packet error occurred at the destination node 102 because the distance from the transmission source node 101 to the destination node 102 is far or the radio wave condition is unstable, even though interception nodes 103 to 109 correctly received the transmission packet from the transmission source node 101 is considered. In the case that the reception condition at the destination node is poor, even if the transmission source node 101 retransmits the transmission packet to the destination node, the possibility that the packet error occurs again at the destination node is high.
Therefore, the interception nodes wait for a reception of the ACK packet from the destination node during a certain waiting time (Tw2), after correctly received the transmission packet from the transmission source node 101. The interception nodes which could not receive the ACK packet until the waiting time Tw2 have passed participate in a procedure for the transmission right acquisition in a random access principle. This procedure is called a backoff procedure. As the result of executing the backoff procedure, an interception node which acquired the transmission right retransmits the datum to the destination node as the proxy transmission node. In this case, it is also assumed that interception nodes do not exist in the neighborhood of the transmission source node and the destination node. For this reason, the transmission source node tries to retransmit the packet as well as other interception nodes. If the proxy transmission by one of the interception nodes will be succeeded, other nodes stop the proxy transmission. In addition, the transmission source node and the interception nodes always try the proxy transmission in the case where there is no reply of the ACK packet from the destination node 102.
Note that, because the backoff procedure is a technology used widely in the wireless LAN system using the CSMA/CA method, the detailed descriptions concerning the backoff procedure are skipped.